Electrically conductive polymeric materials are desirable for many applications including the dissipation of electrostatic charge from parts, electrostatic spray painting and the shielding of electrical components to prevent transmission of electromagnetic waves. The primary method of increasing the electrical conductivity of polymers is to fill them with conductive additives such as metallic powders, metallic fibers, ionic conductive polymers, intrinsically conductive polymeric powder, e.g., polypyrrole, carbon fibers or carbon black. However, each of these approaches has some shortcomings. Metallic fiber and powder enhanced polymers have poor corrosion resistance and insufficient mechanical strength. Further, their density makes high weight loadings necessary. Thus, their use is frequently impractical.
When polyacrylonitrile ("PAN") or pitch-based carbon fiber is added to create conductive polymers the high filler content necessary to achieve conductivity results in the deterioration of the characteristics specific to the original resin. If a final product with a complicated shape is formed by injection molding, uneven filler distribution and fiber orientation tends to occur due to the relatively large size of the fibers, which results in non-uniform electrical conductivity.
Principally because of these factors and cost, carbon black has become the additive of choice for many applications. The use of carbon black, however, also has a number of significant drawbacks. First, the quantities of carbon black needed to achieve conductivity of the polymer are relatively high, i.e. 10-60%. Second, the high morphological "structure" of conductive carbon blacks is subject to breakdown during high shear melt processing. This morphological structure contributes to a reduction of the toughness characteristics to the point where they become too low for many applications. Even when toughness levels are suitable for a given application, the sloughing or rubbing off of the carbon black from the surface of the product may be a problem. Finally, the chemical impurities which are inherent in and result from the typical carbon black manufacturing process, make the use of these materials impractical in, for example, automobile parts.
Carbon fibrils have been used in place of carbon black in a number of applications. For example, it has been recognized that the addition of carbon fibrils to polymers in quantities less than that of carbon black, can be used to produce conductive end products. (See, e.g. Creehan, U.S. application Ser. No. 896,317, filed Jun. 10, 1992, which application is assigned to the same assignee as the present application and is hereby incorporated by reference). It has also been recognized that the addition of carbon fibrils to polymers can be used to enhance the tensile and flexural characteristics of end products. (See, e.g. Goto et al., U.S. application Ser. No. 511,780, filed Apr. 18, 1990, which application is assigned to the same assignee as the present application and is hereby incorporated by reference.)
Carbon fibrils are typically in the form of vermicular tubes with graphitic outer layers disposed substantially concentrically about the cylindrical axis of the fibril. Preferably, the fibrils are substantially free of a pyrolytically deposited thermal carbon overcoat.
Carbon fibrils have a length-to-diameter ratio of at least 5, and more preferably at least 100. Even more preferred are fibrils whose length-to-diameter ratio is at least 1000. The wall thickness of the fibrils is about 0.1 to 0.4 times the fibril external diameter which is preferably between 3.5 and 75 nanometers. In applications where high strength fibrils are needed, e.g., where the fibrils are used as reinforcements, the external fibril diameter is substantially constant over its length.
Prior work by Moy et al., U.S. application Ser. No. 855,122, filed Mar. 18, 1992, which application is assigned to the same assignee as the present application and is hereby incorporated by reference, and Uehara et al., U.S. application Ser. No. 654,507, filed Feb. 23, 1991, which application is assigned to the same assignee as the present application and is hereby incorporated by reference, have disclosed the production of fibril aggregates and their usage in creating conductive polymers.
Moy et al. disclose the production of a specific type of carbon fibril aggregate, i.e. combed yarn, and allude to its use in composites. It does not teach how to use quantities of this aggregate to successfully achieve both conductivity and acceptable notched impact strength or tensile elongation in polymeric compositions. Uehara et al. also disclose the use of fibril aggregates in polymeric materials. The fibril aggregates have a preferred diameter range of 100-250 microns. When these fibril aggregates are added to polymeric compositions and processed, conductivity is achieved. However, notched impact strength is too low for use in most impact situations.